Reactions to Signaling from ECN Support for RTP/RTCP
draft-carlberg-tsvwg-ecn-reactions-03

TSVWG                                                   K. Carlberg
Internet-Draft                                          G11
Intended Status: Informational                          P. O'Hanlon
Expires: April 4, 2013                                  UCL
                                                        Oct 4, 2012

          Reactions to Signaling from ECN Support for RTP/RTCP
              <draft-carlberg-tsvwg-ecn-reactions-03.txt>

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Abstract

   This document presents various responses to Congestion Experience (CE)
   notifications by real time applications that have negotiated end-to-end
   support of Explicit Congestion Notification (ECN).  This document is a
   follow-on effort of [rfc6679], which specifies the signaling used to
   provide ECN support for RTP/RTCP flows.

1. Introduction

   This document presents various responses to Congestion Experience (CE)
   notifications by real time applications that have negotiated end-to-end
   support of Explicit Congestion Notification (ECN).  [rfc6679] defines
   the signaling for support of ECN by RTP based sessions, and also covers
   the case where a set of nodes do not respond to CE notifications.  A
   more detailed discussion about how back-off algorithms can be achieved
   and supported for specific applications is viewed as out of scope of
   that document and may be addressed by a companion document.

1.1 Background

   ECN is a mechanism used to explicitly signal the presence of congestion
   without relying on packet loss.  It was initially designed using a dual
   layer signaling model; negotiation and feedback at the transport layer,
   and downstream notification of congestion at the network layer.  For IP,
   a new two bit field was used to both indicate the successful negotiated
   support for ECN signaling, as well as indicate the presence of
   congestion via the CE flag.  In the case of TCP [rfc3168], a new TCP
   header flag was defined that provides upstream end-to-end indication of
   congestion occurring somewhere along the downstream path.

   There should be no difference in congestion response if ECN-CE marks or
   packet drops are detected.  However it is noted that there MAY be other
   reactions to ECN-CE specified in the future.  Such an alternative
   reaction MUST be specified and considered to be safe for deployment
   under any restrictions specified.  We specify such an alternative in
   this document.

   With respect to ECN for TCP, [rfc3168] specifies an indication of
   congestion, but it does so once per Round Trip Time (RTT).  [rfc6679] is
   an effort that proposes a finer grained notification reflecting a more
   accurate indication of the number of ECN marked packets received within
   one RTT.

1.2  Terminology and Abbreviations

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this

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   document are to be interpreted as described in RFC2119 [RFC2119].

2. Issues

   The initial discussions and presentation of [rfc6679] produced a
   consensus that the specification of signaling was to be done within the
   AVTcore working group, and any subsequent discussion on end-to-end
   reactions to the signaling would be accomplished in the Transport
   Services (TSV) working group.  This draft satisfies the latter effort.

   Another issue that needs to be recognized is that the reactions to CE in
   the context of [rfc6679] are the responsibility of the application.
   This is in contrast to ECN support for TCP, where explicit signaled
   feedback of, and reaction to, CE is kept transparent to the application.
   The issue of placing the feedback responsibility in the application is
   that each application needs to add specific support for that reaction.
   On the other hand, multiple reactions may be considered by the
   application.  For this reason, [rfc6679] states the need for a default
   congestion control reaction that MUST be supported.  Section 3 through 5
   expands on this topic.

3. Congestion Control Algorithms

   The transport of any data flow across the Internet produces a need for
   some form of congestion control to attain a suitable share of the
   capacity of the path through a network.  Most of the existing work on
   realtime congestion control algorithms has been rooted in TCP-friendly
   approaches but with smoother adaptation cycles.  TCP congestion control
   is unsuitable for interactive media for a number of reasons including
   the fact that it is loss-based so it maximises the latency on a path, it
   changes its transmit rate to quickly for multimedia, and favours
   reliability over timeliness. In the case of real time media transport,
   one requires:

       Smoother rate variation: (than for bulk data) to accommodate
               the underlying media flow's characteristics.

       Low latency: Maintaining latencies sufficient to be usable, where
              150ms one way delay is understood to be a good target
              [ITU.G114.2003].

       Fairness: The algorithm must be fair to both itself and other flows

3.1 TCP Friendly Rate Control (TFRC)

   TFRC has a smoother response to congestion than TCP-like approaches,
   thus making it more suitable for real-time interactive multimedia

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   applications.  It has been cited in a number of other documents within
   the IETF for use with UDP and media flows [rfc3714, bcp145] and is
   seeing full and partial deployment in related solutions such as
   Empathy/Farsight, and GoogleTalk [goog1].

   However it should be noted that TFRC is only recommended for real-time
   media use with ECN response. TFRC is not recommended for non-ECN paths
   due to its loss based operation which leads to full queues with
   maximised latencies. It is assumed that ECN markings will usually occur
   with lower queue occupancy and thus lower latency. However it is
   understood that ECN marks may not provide for sufficiently low
   latencies in some situations so other congestion control solutions
   would be preferable.

   [rfc4342] specifies the profile for TFRC for use in the Datagram
   Congestion Control Protocol (DCCP) [rfc4340] for a half connection.  A
   DCCP half connection is defined as application data sent downstream with
   corresponding acknowledgements sent upstream.  These half-connections
   can be realized in the form of one-way pre-recoded media, one-way live
   media, or two-way interactive.  A perceived drawback in this profile
   concerns its application to interactive media that use small packets.
   [RFC4828] is an experimental protocol defining a variation of TFRC used
   to address this drawback and achieve the same bandwidth as a TCP flow
   using packets of size 1500 bytes.

   [rfc6679] is an standard that specifies how RTP flows can
   be supported using the RTP/AVPF profile and the general RTP header
   extension mechanism.

3.2  Related Work

3.2.1 3GPP

   Outside of this previous and on-going work with TFRC, it is understood
   that some parties have issues with the behavior of TFRC under certain
   conditions.  A notable mention of this is made in the 3GPP's document on
   IP Multimedia Subsystem (IMS) Media handling and interaction [TR26.114],
   where it is mentioned:

     "Note that for IMS networks, which normally have nonzero packet loss and
     fairly long round-trip delay, the amount of bitrate reduction specified
     in RFC 3448 is generally too restrictive for video and may, if used as
     specified, result in very low video bitrates already at (for IMS)
     moderate packet loss rates."

   Though it is unclear exactly what the 3GPP community consider as too
   restrictive and whether some alteration of the response may be suitable.
   It should be noted that the 3GPP document only referred to an older

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   version of TFRC defined in [RFC3448]. Given that the current version
   of TFRC [RFC5348] has made significant changes to the idle and data-
   limited responses it is unclear whether their assessment is relevant
   to current TFRC implementations.

   Furthermore the specification [TR26.114] only outlines a rudimentary
   approach to congestion control, providing an example of a 60% back-off
   reaction to loss within an RTCP reporting period. The proposed signalling
   employs Temporary Maximum Media Stream Bit Rate Request (TMMBR)
   [RFC5104] and Codec Mode Request (CMR) [RFC4867] for video and audio
   respectively, which would only provide for very basic rate control
   if used as specified.  We note that [TR26.114] specifies terminal
   behavior, while [TS36.300] specifies base station behaviour, though
   neither specify any standardised congestion control approach.

   It is understood that there are a number of proprietary and patented
   approaches that provide more sophisticated response in the case of
   3G/LTE, but since these are neither endorsed nor standardized this
   document advocates a standardized approach such as TFRC.

   We also acknowledge that there are many congestion control algorithms
   available for implementers to choose from, with a subset that are
   specifically suited to real time media transmission.  However, given a
   variety of real time applications and their various characteristics
   (sender-only broadcast, interactive unicast, etc), we need to expand the
   notion of how back-off can be achieved.  Hence, the focus needs to be on
   an output that would resemble the characteristics of TFRC.

   Within the RTCweb Working Group the need for a more media friendly
   congestion control mechanism has been made apparent.  Currently, TFRC is
   perceived as having deficiencies (e.g. its loss-based design, lack of
   cross-stream congestion control functionality etc) that make it an
   incomplete or insufficient solution for the envisioned RTCWEB media
   flows.  The RTP Media Congestion Avoidance Techniques (rmcat) working
   group has now been formed which aims to lead to the formation of a
   working group on these issues.  The group aims to develop one or more
   congestion control algorithms, associated extensions, and evaluation
   criteria. Furthermore it has been proposed that certain practices, such
   as 'circuit-breaker' conditions, to provide operational limits on
   congestion control algorithms, and feedback messages, may be tackled in
   other groups such as AVTCORE and AVTEXT respectively.

   Thus there is some movement to attempt to develop new algorithms better
   suited to media transport, but these efforts will clearly take a
   considerable time to reach fruition. Whilst TFRC has some perceived
   issues it still provides the best existing solution for media transport.

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3.3 ECN response

   As mentioned above and in accordance to [rfc3168], the actual response
   to the reception of an ECN-CE marked packet MUST normally be the same as
   that of a lost packet.  However there are a number of contexts where one
   may also be interested in more varied approaches.  We expand on this in
   Section 5 below.

4. Application Layer Congestion Response

   Whilst the congestion control algorithm may decide to alter the rate at
   which the application should operate, in the case of media applications
   this process is not as straightforward as the case of bulk data.  The
   different media engines and codecs in use may only have limited
   adaptation ranges, thus, this limitation needs to be a consideration
   when adapting the rate.  Furthermore the application needs to be aware
   of the capability of the specific codecs in terms of their ability to
   switch configuration mid-stream (without loss of fidelity), which may
   impose further limits on the modes of operation.

   One approach for achieving a lower generation of data is through reduced
   sampling of the media (e.g., voice or video).  In the case of video,
   this may also involve slower frame rates.  Specific recommendations that
   describe how applications should respond to congestion in the context of
   supporting the algorithmic characteristics of a congestion control
   algorithm are outside the scope of this document.

5. Other Reactions

   In addition to the activation of congestion control algorithm, other
   reactions can be used or leveraged by an application in response to CE.
   We divide these other potential reactions into two categories: signaling
   and fault tolerance.  We note that these other reactions are considered
   symmetric because they require downstream peer support.  We also point
   out that activation of other reactions represents an example of an
   on-demand and as-needed approach in responding to CE.

5.1 Signaling

5.1.1  RSVP

   The resource Reservation Protocol (RSVP) can be used to signal a desired
   set of path characteristics (e.g., bandwidth, delay) in response to CE
   feedback [rfc2205].  Its operation is based on the use of PATH messages
   sent downstream hop-by-hop from the source to a destination that specify
   requested forwarding characteristics.  In return, the destination sends
   a hop-by-hop RESV message upstream towards the source confirming the
   resources that have been reserved for that flow.

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   [rfc3181] defines a priority policy element that specifies both an
   allocation and defending priority.  This dual specification supports the
   use of preemption of existing reservations.  [draft-priority-rsvp] is a
   work-in-progress that defines a new policy element that only conveys
   priority during reservation establishment.  This latter effort also
   presents several reservation models, including one that describes
   engineered resources set aside for priority users.

5.1.2 Differentiated Services

   Unlike RSVP and its use of a separate signaling mechanism to reserve
   resources, Differentiated Services (diff-serv) uses code points within
   the IP header to convey the forwarding behavior of that packet
   [rfc2474].  This may range from various drop precedence values to a code
   point that signifies low delay and low loss (i.e., characteristics
   attributed to real time flows).

   As in the case of RSVP, applications could rely on the reception of CE
   feedback to initiate a subsequent setting of diff-serv code points to
   provide additional protection or explicit association of forwarding
   characteristics of a given flow of packets.  In addition, the setting of
   diff-serv code points would be done on an as-needed basis in reaction to
   CE feedback.  Recommendations concerning specific diff-serv values are
   outside the scope of this document.

5.2 Fault Tolerance

   Fault tolerance is another category of reactions that may be used by
   applications in response to CE feedback.  In some cases, these efforts
   may contribute to an increase in traffic load in order to add protection
   and resiliency to a flow.

   Redundant Transmissions: This approach is based on a source sending
   duplicate payloads that can be used to compensate for lost packets.
   Given that ECN marks the packet and forwards it towards the destination
   (instead of dropping it), this approach can be considered extreme in
   terms of being network unfriendly.  Its positive value may emerge in
   cases where a path has several downstream congestion points.  However,
   its actions of producing redundant packets still associates a high
   measure of greedy use of resources.

   Application Layer Forward Error Correction (FEC): This approach also
   adds additional overhead to the flow in order to compensate for
   potential packet loss.  And as the case of redundant transmissions, the
   value of this approach is probably better realized when there exists
   multiple downstream congestion points.  However, the impact of the
   overhead is minimized by having one (or a few) additional packet(s) used
   to compensate for the loss of a set of packets.

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   Codec Swapping: This approach involves changing codecs to either reduce
   load or achieve an improvement in compensating for lost packets.

5.3  Alternative Reaction for Emergency Communications

   As mentioned in [rtp-ecn], the default reaction on the reception of
   these ECN-CE marked packets MUST be to provide the congestion control
   algorithm with a congestion notification that triggers the algorithm to
   react as if packet loss had occurred.  There MAY be an alternative
   reaction if it is considered safe for deployment.  An example of the
   need for an alternative reaction would be the case of Emergency
   Telecommunications Service (ETS) [rfc3689, rfc4190], where an
   improvement in QoS or a higher probability of session establishment and
   forwarding of traffic is of high interest.

   It is proposed that certain authorized ETS flows may be permitted to
   employ either a substantially less aggressive back-off algorithm than
   the default algorithm, or some level of exemption from reacting to ECN
   marked packets.  This alternative reaction will benefit these flows as
   the marks would normally be considered as equivalent to lost packets,
   which would effectively increase the loss level, which in turn will
   generally result in the reduction of flow rate. This applies to all
   flows that utilize some form of the rate control that is inversely
   proportional to the loss rate, which includes TCP-like algorithms or
   equation-based approaches.

   Simulations of the use of ECN exemption with TFRC and have found that it
   has limited effect on the normal flows with low numbers of exempt
   flows. A half-dumbbell network was used with a RED router queue
   configured using the settings recommended by Sally Floyd. The candidate
   flows are 1Mbit/s each with a backhaul 100Mbit/s link. In the standard
   case where 1% of flows would be exempt the remaining flows achieve
   99.99% of the bandwidth that they would achieve without the presence of
   the exempt flows. This is what would be expected from the simple
   calculation of the allocation, given that the exempt flows achieve their
   full rate (1Mbit/s); With 100 normal plus 1 exempt flow, assuming that
   the except flow uses 1Mbit/s, the remaining capacity is 99Mbit/s which
   is divided between the 100 normal flows.  Whilst when 101 normal flows
   are run over the 100Mbit/s link they would have to share it evenly, so
   it works out thus: ((99/100)/(100/101))*100=99.99%. In the case of 5%
   exempt flows then the proportion is very slightly lower at
   ((95/100)/(100/105))*100=99.75%.  Both these calculations are borne out
   in the simulation runs.

   The level of exemption employed can be altered in a number of ways. Two
   simple approaches would be to either set a threshold number of ECN
   marked packets that could be considered as a loss, and another approach
   would be to set a percentage threshold of ECN marked packet that would

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   be considered as a loss.

   It should be noted that in the simulations the end-to-end delay of the
   packets within the flows was monitored and the relative delay of the
   exempt flows apparently rises somewhat when exemption is
   enacted. However what is actually occurring is that the 'normal' flows
   are reducing their throughput and are thus reducing their latency
   somewhat. There is normally some limited latency when using loss-based
   techniques such as TFRC because it fills the queues to ascertain the
   link capacity and maintains that level of delay throughout a
   session. However the level of latency is clearly limited by the queue
   sizes in the network and on media specific links these queue sizes are
   typically quite small, so the resulting latency is limited.

   Furthermore in the case where media flows employing TFRC, or any other
   congestion control algorithm (e.g. delay-based), are sharing a
   bottleneck link with TCP flows then the queues will be filled by the TCP
   flows and the latency will be kept near or at a their maximum despite
   any other flows.

6.  IANA Considerations

   This document requires no actions from IANA.

7.  Security Considerations

   The reliance on accurate and un-modified RTCP information means that
   SRTP needs to be used, or any other mechanism that helps prevent
   modification of RTCP feedback packets.

8. Acknowledgements

   TBD

9.  References

9.1 Normative

   [rfc2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

   [rfc2205] Braden, B., et. al., "Resource ReSerVation Protocol (RSVP)
             -- Version 1 Functional Specification", RFC 2205, September
             1997

   [rfc2209] Braden, R., L. Zhang, "Resource Reservation Protocol
             (RSVP) -- Version 1 Message Processing Rules", RFC2209

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             September 1997

   [rfc2474] Nichols, K., et. al., "Definition of the Differentiated
             Services Field in the IPv4 and IPv6 Headers", RFC 2474,
             December 1998

   [rfc3168] Ramakrishnan, K,. et. al., "The Addition of Explicit
             Congestion Notification (ECN) to IP", RFC 3168,
             September, 2001

   [rfc3181] Herzog, S., "Signaled Preemption Priority Policy Element",
             RFC 3181, October 2001

   [rfc3448] Handley, M., et. al., "TCP Friendly Rate Control (TFRC):
             Protocol Specification", RFC 3448, January 2003

   [rfc4867] Sjoberg, J., et. al., "RTP Payload Format and File Storage
             Format for the AMR and AMR-WB Audio Codecs", RFC 4867,
             April 2007

   [rfc5104] Wenger, S., et. al., "Codec Control Messages in the RTP
             Audio-Visual Profile with Feedback (AVPF)", RFC 5104,
             February 2008

   [rfc6679] Westerlund, M., et. al., "Explicit Congestion
             Notification (ECN) for RTP over UDP", RFC 6679,
             IETF, July 2012

9.2 Informative

   [draft-rtp-tfrc] Gharai, L., C. Perkins, "RTP with TCP Friendly Rate
                    Control", work-in-progress, Sept 2011

   [Goog1] http://code.google.com/apis/talk/call_signaling.html

   [tr26.114] "IMS; Multimedia telephony; Media Handling and
              Interaction", 3GPP, version 10, April 2011

   [ts36.300] "E-UTRA and E-UTRAN Overall Description, Stage 2",
              3GPP, Release 10, September, 2011

   [RFC4340] Kohler, E., et. al, Datagram Congestion Control
             Protocol (DCCP), RFC4340, March 2006

   [RFC4342] Floyd, S., et. al., "Profile for DCCP Congestion
             Control ID 3: TFRC", RFC 4342, March 2006

   [RFC4828] Floyd, S., E. Kohler, "TFRC: The Small Packet

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             Variant", RFC 4828, April 2007

   [rfc3689] Carlberg, K., Atkinson, R., "General Requirements for
             Emergency Telecommunications Service (ETS)", RFC 3689,
             February 2004

   [rfc4190] Carlberg, K. et, al., "Framework for Supporting
             Emergency Telecommunications Service (ETS) in
             IP Telephony", RFC 4190, November 2005

   [rfc3714] Floyd, S., Kempf, J., "IAB Concerns Regarding Congestion
             Control for Voice Traffic in the Internet", RFC 3714,
             March 2004

   [bcp145]  Eggert, L., Fairhurst, G., "Unicast UDP Usage Guidelines
             for Application Designers", RFC 5405, BCP 145, November 2008

   [ITU.G114.2003]
             International Telecommunications Union, "One-way
             transmission time", ITU-T Recommendation G.707, May 2003.

Author's Addresses

   Piers O'Hanlon
   University of Oxford
   Oxford Internet Institute
   1 St Giles
   Oxford  OX1 3JS
   United Kingdom

   Email: piers.ohanlon@oii.ox.ac.uk

   Ken Carlberg
   G11
   1600 Clarendon Blvd
   Arlington  VA
   USA

   Email: carlberg@g11.org.uk

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